These are services where you can list or launch your startup to a broad, preexisting community in an attempt to encounter new user types. Experimentally, systematically launching to everyone on this list of communities will yield a few hundred to several thousand signups.

It's important to note that these are typically unqualified sources of traffic – early adopter communities and similar – so when analyzing the outcomes here, you should be looking for patterns in feedback from users to find reproducible user personas, versus treating this as a scalable way to get growth.

The "A" ranked places on Jesse's list are worth starting with, and summarized below along with his comments. (I wouldn't bother with anything past "B"– you should be more focused than that.)

Reddit Programming. "Very strong community. Great leads if your product is for devs."